Digital cameras, such as the Kodak DCS ProBack 645M camera, capture images of a scene using a solid state image sensor, and store the resulting image data on a removable memory device, such as a PCMCIA type III hard drive. Thumbnails of the captured images can be displayed on the camera's color LCD screen. Since scenes can have a wide range of illumination levels, these cameras include automatic exposure controls in order to adjust the camera lens f/number and exposure time to compensate for the scene illumination level. However, because of the varying reflectance levels of objects within a scene, and the limited dynamic range of image sensors, such automatic exposure control often produces unacceptable results for professional photographers.
As a result, such digital cameras include manual exposure overrides. The photographer can review a captured image on the camera's LCD image display to determine if the captured scene appears lighter or darker than desired, adjust the exposure settings, and can take a second picture. However, because of the small size and limited picture quality of the LCD display, it is impossible to make critical exposure judgements using the displayed image. As a result, this method is useful only for providing very coarse exposure adjustments.
Once the images are captured by a digital camera (such as the Kodak DCS 620 or Kodak DCS ProBack 645M cameras), they can be downloaded to a computer and processed and displayed. For example, an image processing program such as Photoshop version 6.0 by Adobe Systems Inc., San Jose, Calif. can be used to display and edit a captured image. Photoshop version 6.0 includes an “info tool” which displays the RGB code values of a particular pixel when the user lingers the cursor over a particular image area. The displayed values are the RGB code values of the processed pixels from the camera, which may include many types of non-linear quantization and processing. As a result, it is not possible to easily relate the RGB code values to the sensor exposure values of the camera when the scene was captured. Furthermore, these displayed code values are available only after the images are downloaded to the computer, and not as the images are being captured.
U.S. Pat. No. 5,414,537, issued May 9, 1995, inventors Omuro et al., discloses a color image processing method and apparatus in which a color image of an object is processed so that the processed image accurately portrays the colors of the object regardless of varying exposure conditions. An object and a color chart that is comprised of a plurality of color chips having known colorimetric parameters are imaged under the same predetermined exposure conditions. Then, colorimetric parameters are determined for each imaged color chip of the imaged color chart. A correlation, between the known and the determined colorimetric parameters, is computed and, on the basis of the captured correlation, the color of the imaged object is corrected. This technique is disadvantageous because it requires user input, is effected in a computer separate from the camera and only corrects for color anomalies.
What is needed is a digital camera that provides an easy way for the photographer to understand the sensor exposure values for different areas of a scene as the scene is captured, so that any desired exposure corrections can be made automatically and the scene can be immediately recaptured.